Filter using piezoelectric film bonded to high resistivity silicon substrate with trap-rich layer

ABSTRACT

Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a trap-rich region adjacent to a surface and a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The single-crystal piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm.

RELATED APPLICATION INFORMATION

This patent claims priority to provisional patent application62/951,452, filed Dec. 20, 2019, entitled PIEZOELECTRIC FILM BONDED TOHIGH RESISTIVITY SILICON HAVING TRAP-RICH LAYER FOR RF FILTERS. Thispatent is a continuation in part of application Ser. No. 16/438,121.filed Jun. 11, 2019, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR, which is a continuation-in-part of application Ser. No.16/230,443, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, now U.S. Pat. No. 10,491,192, which claims priorityfrom the following provisional patent applications: application62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR);application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR(XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR;and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. All of theseapplications are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE′ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave BAW) resonators, film bulk acousticwave resonators (FBAR), and other types of acoustic resonators. However,these existing technologies are not well-suited for use at the higherfrequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 5 is schematic cross-sectional view and two detailedcross-sectional views of a filter using XBARs.

FIG. 6 is a chart comparing the input/output transfer functions offilters fabricated using high resistivity and low resistivitysubstrates.

FIG. 7 is a flow chart of a process for fabricating a filter on apiezoelectric plate bonded to a silicon substrate having a trap-richlayer.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front and back surfaces 112, 114.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including rotated Z-cut and rotated YXcut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers.

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 (as shown subsequently in FIG. 3A and FIG. 3B). The cavity 140 maybe formed, for example, by selective etching of the substrate 120 beforeor after the piezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate that spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular cross sectionwith an extent greater than the aperture AP and length L of the IDT 130.A cavity of an XBAR may have a different cross-sectional shape, such asa regular or irregular polygon. The cavity of an XBAR may more or fewerthan four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT. Similarly, the thickness ofthe fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100of FIG. 1. The piezoelectric plate 110 is a single-crystal layer ofpiezoelectrical material having a thickness ts. ts may be, for example,100 nm to 1500 nm. When used in filters for LTE™ bands from 3.4 GHZ to 6GHz (e.g. bands 42, 43, 46), the thickness ts may be, for example, 200nm to 1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2, the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. tfd and tbd maybe, for example, 0 to 500 nm. tfd and tbd are typically less than thethickness ts of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The IDT fingers 238 may be aluminum, substantially aluminum alloys,copper, substantially copper alloys, beryllium, gold, or some otherconductive material. Thin (relative to the total thickness of theconductors) layers of other metals, such as chromium or titanium, may beformed under and/or over the fingers to improve adhesion between thefingers and the piezoelectric plate 110 and/or to passivate orencapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness is of the piezoelectric plate 110. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3 shows an alternative cross-sectional view along the section planeA-A defined in FIG. 1. In FIG. 3, a piezoelectric plate 310 is attachedto a substrate 320. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the substrate. The cavity 340does not fully penetrate the substrate 320. Fingers of an IDT aredisposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around a large portion of aperimeter 345 of the cavity 340. For example, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around at least50% of the perimeter of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. An RF voltage is applied to the interleaved fingers 430. Thisvoltage creates a time-varying electric field between the fingers. Thedirection of the electric field is lateral, or parallel to the surfaceof the piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric field is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites a shear-mode acoustic mode, inthe piezoelectric plate 410. In this context, “shear deformation” isdefined as deformation in which parallel planes in a material remainparallel and maintain a constant distance while translating relative toeach other. A “shear acoustic mode” is defined as an acoustic vibrationmode in a medium that results in shear deformation of the medium. Theshear deformations in the XBAR 400 are represented by the curves 460,with the adjacent small arrows providing a schematic indication of thedirection and magnitude of atomic motion. The degree of atomic motion,as well as the thickness of the piezoelectric plate 410, have beengreatly exaggerated for ease of visualization. While the atomic motionsare predominantly lateral (i.e. horizontal as shown in FIG. 4), thedirection of acoustic energy flow of the excited primary shear acousticmode is substantially orthogonal to the surface of the piezoelectricplate, as indicated by the arrow 465.

Considering FIG. 4, there is essentially no electric field immediatelyunder the IDT fingers 430, and thus acoustic modes are only minimallyexcited in the regions 470 under the fingers. There may be evanescentacoustic motions in these regions. Since acoustic vibrations are notexcited under the IDT fingers 430, the acoustic energy coupled to theIDT fingers 430 is low (for example compared to the fingers of an IDT ina SAW resonator), which minimizes viscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. Thus, high piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters withappreciable bandwidth.

FIG. 5 shows a schematic cross-sectional view and two detailedcross-sectional views of a filter 500 using XBARs. A piezoelectric plate510 is attached to a substrate 520. An optional dielectric layer 525 maybe sandwiched between the piezoelectric plate 510 and the substrate 520.A portion of the piezoelectric plate 510 forms a diaphragm 515 spanninga cavity 540 in the substrate. As shown, the cavity 540 does not fullypenetrate the substrate 520. Alternatively, the cavity 540 may penetratethe substrate as shown in FIG. 1. Fingers of an IDT are disposed on thediaphragm 515. Two conductors 550 and 555 are formed on the surface ofthe piezoelectric plate 510 at a location removed from the cavity 540.The two conductors 550, 555 may be signal conductors interconnectingXBARs and/or other components of the filter 500. The conductors 550 and555 may be a signal conductor and a ground conductor. While FIG. 5 onlyshows a single XBAR and two conductors, a filter may include multipleXBARs and more than two signal and ground conductors.

A preferred material for the substrate 520 is silicon. Silicon wafersare readily available and inexpensive. Further, processes and equipmentfor handling silicon wafers are well developed. However, silicon is asemiconductor material. Silicon wafers may be doped, or loaded withimpurities, to have a desired bulk resistivity. Undoped, or intrinsic,silicon wafers can form a conductive inversion layer along the boundarybetween the silicon and some other material, such as along the boundaryof the silicon wafer 520 and the dielectric layer 525 of the filterdevice 500. If the dielectric layer 525 is not present, the inversionlayer may form along the boundary between the silicon wafer 520 and thepiezoelectric plate 510.

As shown in Detail A of FIG. 5, conductors 550 and 555 are capacitivelycoupled to the substrate 520 through the piezoelectric plate 510 and thedielectric layer 525, if present. If the substrate 520 is conductive, orif a conductive inversion layer is formed in the substrate 520, theconductors 550, 555 will be effectively connected, at RF frequencies, bya parasitic resistance 560. Power dissipated in the resistance 560 willcontribute to the insertion loss of the filter 500.

FIG. 6 shows an exaggerated example of the degradation of a filter dueto substrate conductivity. FIG. 6 is a plot of the magnitude of S21 (theinput-output transfer function) versus frequency for two filters thatare identical except for the choice of substrate material. The solidline 610 is a plot of S21 for a filter fabricated on a nearly insulatingsilicon substrate with a bulk resistivity of 5000 ohm-cm. The dashedline 620 is a plot of S21 for a filter fabricated on conductive siliconsubstrate with a bulk resistivity of 15 ohm-cm. Both plots are based onsimulation of the filter using a finite element method. The differencein the two filters is evident. Substrate conductivity decreases S21(i.e. increases insertion loss) by 6 dB or more in the filter passband.The effect of a conductive inversion layer in a high resistivitysubstrate will have a less dramatic, but still significant, effect oninsertion loss.

Referring back to FIG. 5, detail B shows a cross-sectional view of aportion of a filter formed on a substrate 520 including a highresistivity silicon wafer 522 and a trap-rich region 524. The trap richregion 524 may be a region within the high resistivity silicon wafer 522or a layer formed on a surface of the high resistivity silicon wafer522. In either case, the trap-rich region is immediately adjacent thedielectric layer 525 or the piezoelectric plate 510 if the dielectriclayer 525 is not present. The trap-rich region 522 has an abundance oftraps that capture free carriers and reduce carrier lifetime to anextent that the conductivity of the trap-rich region approaches zero.

A trap-rich region may be formed within a silicon substrate byirradiating the surface of the substrate with neutrons, protons, orvarious ions (silicon, argon, nitrogen, neon, oxygen, etc.) to createdefects in the crystalline structure of the substrate. Alternatively, atrap-rich region may be formed within a silicon substrate by introducingdeep trap impurities such as gold, copper, or other metal ions. Suchimpurities may be introduced by ion implantation, diffusion, or someother method. The trap-rich region may be formed by a combination ofthese techniques. When the dielectric layer 525 is included in thefilter 500, the trap-rich region 524 may be formed before the dielectriclayer is formed on the substrate 520. Alternatively, the trap-richregion 524 may be form by ion implantation through the dielectric layer525.

A trap-region layer may be formed on a silicon substrate by depositing alayer of trap-rich material such as amorphous silicon or polysilicon(polycrystalline silicon). When the trap-rich region is polysilicon, theaverage grain size of the polysilicon should be substantially smallerthan the minimum spacing between electrodes 550, 555. The thickness ofthe trap rich region formed on or within a high resistivity siliconsubstrate should be greater than the thickness of an inversion layerthat may form in the absence of the trap-rich layer.

Description of Methods

FIG. 7 is a simplified flow chart showing a process 700 for making afilter incorporating XBARs and a substrate with a trap-rich region. Theprocess 700 starts at 705 with a substrate and a plate of piezoelectricmaterial and ends at 795 with a completed XBAR or filter. The flow chartof FIG. 7 includes only major process steps. Various conventionalprocess steps (e.g. surface preparation, cleaning, inspection, baking,annealing, monitoring, testing, etc.) may be performed before, between,after, and during the steps shown in FIG. 7.

The flow chart of FIG. 7 captures three variations of the process 700for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 715A, 715B, or 715C.Only one of these steps is performed in each of the three variations ofthe process 700.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate. The piezoelectric plate may be rotated Z-cut orrotated YX cut lithium niobate. The piezoelectric plate may be someother material and/or some other cut. The substrate may preferably behigh resistivity silicon. The substrate may be some other material thatallows formation of deep cavities by etching or other processing.

At 710, a trap-rich region may be formed on the substrate. The trap-richregion may be formed within a silicon substrate by irradiating thesurface of the substrate with neutrons, protons, or various ions(silicon, argon, nitrogen, neon, oxygen, etc.) to disrupt thecrystalline structure of the substrate. The trap-rich region may beformed within a silicon substrate by introducing deep trap impuritiessuch as gold, copper, or other metal ions. Such impurities may beintroduced by ion implantation, diffusion, or some other method. Thetrap-rich region may be formed by a combination of these techniques.When the dielectric layer 525 is included in the filter 500, thetrap-rich region 524 may be formed before the dielectric layer is formedon the substrate 520. Alternatively, the trap-rich region 524 may beformed by ion implantation through the dielectric layer 525,

Alternatively, at 710, a trap-rich region may be formed on the siliconsubstrate by depositing a layer of trap-rich material such as amorphoussilicon or polysilicon. When the trap-rich region is polysilicon, theaverage grain size of the polysilicon should be substantially smallerthan the minimum spacing between electrodes 550, 555.

In all cases, the thickness of the trap rich region formed at 710 shouldbe greater than the thickness of an inversion layer that may form in theabsence of the trap-rich region.

In one variation of the process 700, one or more cavities are formed inthe substrate at 715A before the piezoelectric plate is bonded to thesubstrate at 720. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 715A will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3A or FIG. 3B.

At 720, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 730 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metals may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 730 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 730 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 740, a front-side dielectric layer may be formed by depositing one ormore layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 700, one or more cavities areformed in the back side of the substrate at 715B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 700, a back-side dielectric layermay be formed at 750. In the case where the cavities are formed at 715Bas holes through the substrate, the back-side dielectric layer may bedeposited through the cavities using a conventional deposition techniquesuch as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 700, one or more cavities in theform of recesses in the substrate may be formed at 715C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 715C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3.

In all variations of the process 700, the filter device is completed at760. Actions that may occur at 760 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 760is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at795.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substrate having a trap-rich region adjacent to a surface; a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity formed in the substrate; and an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm, wherein the substrate comprises a single-crystal silicon plate, and wherein the trap-rich region is a portion of the silicon plate that has been irradiated to create defects in the crystalline structure.
 2. The device of claim 1, wherein the diaphragm is contiguous with the piezoelectric plate around at least 50% of a perimeter of the cavity.
 3. The device of claim 1, wherein the trap-rich region is a layer of trap-rich material formed on a surface of the silicon plate.
 4. The device of claim 3, wherein the trap-rich material is amorphous silicon or polycrystalline silicon.
 5. The device of claim 1, wherein a depth of the trap-rich region is greater than a depth of an inversion layer that would form in the substrate in the absence of the trap-rich region.
 6. An acoustic resonator device comprising: a substrate having a trap-rich region adjacent to a surface; a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity formed in the substrate; and an interdigital transducer (IDT) formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm, wherein the substrate comprises a single-crystal silicon plate, and wherein the trap-rich region is a portion of the silicon plate containing deep trap impurities.
 7. The device of claim 6, wherein the trap-rich region is a layer of trap-rich material formed on a surface of the silicon plate.
 8. The device of claim 7, wherein the trap-rich material is amorphous silicon or polycrystalline silicon.
 9. The device of claim 6, wherein a depth of the trap-rich region is greater than a depth of an inversion layer that would form in the substrate in the absence of the trap-rich region.
 10. A filter device, comprising: a substrate having a trap-rich region adjacent to a surface; a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate, portions of the single-crystal piezoelectric plate forming one or more diaphragms spanning respective cavities in the substrate; and a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, interleaved fingers of each of the plurality of IDTs disposed on one of the one or more diaphragms, wherein the single-crystal piezoelectric plate and all of the IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective shear primary acoustic mode within the respective diaphragm, wherein the substrate comprises a single-crystal silicon plate, and wherein the trap-rich region is a portion of the silicon plate that has been irradiated to create defects in the crystalline structure.
 11. The filter device of claim 10, wherein each of the one or more diaphragms is contiguous with the piezoelectric plate around at least 50% of a perimeter of the respective cavity.
 12. The filter device of claim 10, wherein the trap-rich region is a layer of trap-rich material formed on a surface of the silicon plate.
 13. The filter device of claim 12, wherein the trap-rich material is amorphous silicon or polycrystalline silicon.
 14. The filter device of claim 10, wherein a depth of the trap-rich region is greater than a depth of an inversion layer that would form in the substrate in the absence of the trap-rich region.
 15. A filter device, comprising: a substrate having a trap-rich region adjacent to a surface; a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate, portions of the single-crystal piezoelectric plate forming one or more diaphragms spanning respective cavities in the substrate; and a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators, interleaved fingers of each of the plurality of IDTs disposed on one of the one or more diaphragms, wherein the single-crystal piezoelectric plate and all of the IDTs are configured such that a respective radio frequency signal applied to each IDT excites a respective shear primary acoustic mode within the respective diaphragm, wherein the substrate comprises a single-crystal silicon plate, and wherein the trap-rich region is a portion of the silicon plate containing deep trap impurities.
 16. The filter device of claim 15, wherein the trap-rich region is a layer of trap-rich material formed on a surface of the silicon plate.
 17. The filter device of claim 16, wherein the trap-rich material is amorphous silicon or polycrystalline silicon.
 18. The filter device of claim 15, wherein a depth of the trap-rich region is greater than a depth of an inversion layer that would form in the substrate in the absence of the trap-rich region.
 19. A method of fabricating an acoustic resonator device on a substrate having a trap-rich region adjacent to a surface, the method comprising: attaching a back surface of a single crystal piezoelectric plate to the surface of the substrate; forming a cavity in the substrate such that a portion of the single-crystal piezoelectric plate forms a diaphragm spanning the cavity; and forming an interdigital transducer (IDT) on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm, wherein the cavity is formed prior to attaching the single crystal piezoelectric plate to the surface of the substrate.
 20. The method of claim 19, wherein the diaphragm is contiguous with the piezoelectric plate around at least 50% of a perimeter of the cavity.
 21. A method of fabricating an acoustic resonator device on a substrate having a trap-rich region adjacent to a surface, the method comprising: attaching a back surface of a single crystal piezoelectric plate to the surface of the substrate; forming a cavity in the substrate such that a portion of the single-crystal piezoelectric plate forms a diaphragm spanning the cavity; and forming an interdigital transducer (IDT) on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, the single-crystal piezoelectric plate and the IDT configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm, wherein the cavity is formed after attaching the single crystal piezoelectric plate to the surface of the substrate.
 22. The method of claim 21, wherein the diaphragm is contiguous with the piezoelectric plate around at least 50% of a perimeter of the cavity. 